This invention relates to the art of geophysical prospecting in general, to the art of radioactivity well logging in particular, and more particularly, to improved methods and apparatus for background subtracting for pulsed neutron logging of earth formations traversed by a borehole.
It is known in the prior art to irradiate earth formations with neutrons from a neutron source, for example, a D-T accelerator. These neutrons participate in elastic collisions with the atoms contained in the various layers of the earth formations. Secondary radiations resulting from the bombardment of the formations by the primary radiation are detected following the cessation of the irradiation. The measurement of the decline of the thermal neutron population provides a means for determining a property of the material surrounding the borehole, the macroscopic capture cross-section [Sigma (.SIGMA.)] of the formation.
In U.S. Pat. No. 3,379,882, issued to A. H. Youmans and assigned to the assignee of the present invention, there is described and illustrated a system for radioactivity well logging wherein the earth formations are irradiated with neutrons from a periodically varying source operating at a repetition rate of the order of magnitude of hundreds or thousands of cycles per second, being thus alternately on and off for periods of hundreds of microseconds. A detecting system is synchronized with the source to operate while the source is in an off state. The radiation detected may be either slow neutrons or gamma rays which result from thermal neutron capture; in either instance the signal is related to the population of thermal neutron in the earth formations surrounding the borehole. By measuring the number of thermal neutrons or gamma rays produced by the thermal neutrons present at any particular time, the rate of decay of the thermal neutron population may be measured. The radioactivity well log which is normally conducted according to this described well logging system comprises a pair of counting rate curves plus a curve indicative of the rate of decline of the neutron population which is a function of ratio of the two counting rate curves. The rate of decline curve is calibrated to record the macroscopic thermal neutron capture cross-section of the earth formations.
In U.S. Pat. No. 3,706,884, also assigned to the assignee of the present invention, there is described a system for using three detection gates following each neutron burst and associated circuits for substantially eliminating radiation background count in pulsed neutron well logging. The third detection gate is used to obtain a relative measurement of background counting rate. The background measurement can then be used to correct the signal measurement counting rates of the two detection time intervals.
Yet another method and system of measuring the macroscopic thermal neutron capture cross-section of the earth formations is described in U.S. Pat. No. 4,046,764, issued to G. L. Marquis and assigned to the assignee of the present invention. All of the detected pulses within a selected time interval following the neutron bursts are used to determine a single timing point. The timing point and the known absorption mode are used to compute the macroscopic cross-section. Since all detected data are used to determine the single value, the computed values of macroscopic cross-section are statistically more accurate than values obtained by prior systems. While the statistical reliability of this system is superior to the prior systems, a suitable method of compensating for timing background has been lacking.
Accordingly, the present invention overcomes these difficulties by providing method and apparatus for substantially eliminating the effect of background upon the timing measurement derived using a single signal detection interval.